Motor oil is a type of liquid oil used for lubrication by various kinds of internal combustion engines. Other benefits from using motor oil include cooling by carrying heat away from moving engine parts and often include cleaning and corrosion inhibition in internal combustion engines. Motor oil is used as a lubricant in various kinds of internal combustion engines in which there are contacting parts which move against each other at high speeds, often for prolonged periods of time. Hydraulic oils are used for automotive and other braking systems. Such rubbing motion causes friction, absorbing otherwise useful power produced by the motor and converting the energy to useless heat. Lubricating and hydraulic oils makes a film between surfaces of parts moving against each other so as to minimize direct contact between them decreasing friction, wear, and production of excessive heat. Coating metal parts with oil also keeps them from being exposed to oxygen, which inhibits their oxidation at elevated operating temperatures. In summary, in ideal situation, a lubricant will physically separate these asperities with an oil film. This is called full fluid film lubrication. When the proper lubricant is used and the proper load is applied, the asperities are not in contact and no wear occurs.
Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives added as needed to improve the properties. Among various oil additives, anti-oxidants, the viscosity index improvers, detergents and dispersants are the most important to help keep the engine clean by minimizing sludge buildup, corrosion inhibitors, and alkaline additives to neutralize acidic oxidation products of the oil. For examples, a certain amount of anti-oxidants is usually charged to the base oil in order to stabilize the base oils or other additives from degradation via oxidation. Commercial oils also have additives like detergents and dispersants, the former is to neutralize the acids in oils generated during service while the later is to stabilize the particulates in oils from depositing onto the surface. Moreover, most commercial oils have a minimal amount of zinc dialkyldithiophosphate as an anti-wear additive to protect contacting metal surfaces with zinc and other compounds in case of metal to metal contact. The quantity of zinc dialkyldithiophosphate is limited to minimize adverse effect on catalytic converters.
There are also other additives available commercially which can be added to the oil by the user for purported additional benefit. Some of these additives include: Zinc dialkyldithiophosphate (ZDDP) additives, which typically also contain calcium, are available to consumers for additional protection under extreme-pressure conditions or in heavy duty performance situations. ZDDP and calcium additives are also added to protect motor oil from oxidative breakdown and to prevent the formation of sludge and varnish deposits.
During services in engines, there is inevitably some exposure of the oils to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation. Also, since no solid surface is perfectly smooth, opposing friction surfaces have peaks called asperities that come in contact with each other. As is known, rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against the part surfaces causing erosion and wear. The oil filter removes many of the particles, but eventually the oil filter gets filled up. Moreover, the motor oil and especially the additives also undergo oxygen, thermal and mechanical degradation. For these reasons, the oil quality in engine will continuously deteriorate till it no longer functions well as a lubricant and, therefore, the oil and the oil filter need to be periodically replaced.
The vehicle manufacturer specifies which grade of oil should be used for the vehicles it produces. The manufacturer also specifies how often the oil changes should be made. For example, most people in the United States believe that a common oil change frequency should be every 3000 miles or every 3 months, whichever comes sooner. This 3000 mile oil change interval has been relentlessly promoted by oil changing companies for decades. It had a scientific basis when engines used non-multi-weight, non-detergent oil. It no longer has any scientific basis, but it is still being promoted by certain entities, most notably the oil change industry in the United States (including car dealerships). Indeed, studies have shown more wear occurs with fresher (1000-2000 mile) oil. This is attributed to additives re-establishing themselves, TBN converging, and filters becoming more efficient. Most manufacturers recommend oil change intervals of 6,000 miles or more for modern cars and heavy duty trucks. In Europe, by contrast, where the influence of oil companies has been much less, oil is typically changed only at a major service interval, between 10,000 and 15,000 miles for a modern car and heavy duty truck. For convenience, the oil filter is usually also replaced at the time the oil is changed.
All the above mentioned used oil replacement mechanism are approaches based on empirical correlation derived from correlation between oil properties analysis results and engine performance test. A number of oil analysis experiments are now carried out in oil analysis labs, examples of which are described below.
First, to maintain a lubricating film between moving parts, the viscosity of oils must be high enough, but low enough that the oil can flow around the engine parts satisfactorily to keep them well coated under all conditions. The Viscosity Index is a measure of how much the oil's viscosity changes as temperature changes. A higher viscosity index indicates the ‘viscosity’ changes less with temperature than a lower viscosity index.
Second, another test done on oil is to determine the Total Base Number (TBN), which is a measurement of the reserve alkalinity of an oil to neutralize acids. The resulting quantity is determined as mg KOH/(gram of lubricant). Analogously, Total Acid Number (TAN) is the measure of a lubricant's acidity.
Finally, still other tests include zinc, phosphorus, or sulfur content, and testing for excessive foaming. Among the various oil analysis, it is recommended that oil viscosity, oil acidity (total acidity number, TAN, or total base number, TBN), particulates in oils and oil oxidation onset time, etc., are most important parameters for oil quality monitoring purpose.
In the prior art of oil quality measurements, “external oil analysis” is the most common mode. This means that one need to get some oil samples from engines and send it to oil analysis labs for measurement. This non real-time measurement of oil quality has several disadvantages and is typically regarded as a “protective approach” for engine maintenance. Recently, it is highly recommended to take the “proactive approach” of engine maintenance based on real time oil quality monitoring. This invention aims to provide a real-time approach for oil acidity (TAN & TBN) measurement for the purpose of “proactive” engine maintenance.